In a fluid handling device, such as a valve or a pump, where a moving stem or shaft extends through a wall of the device, a seal is required at that point to prevent the fluid from leaking from the device. This is often accomplished by packing a compressed packing material around the shaft, and containing the packing material in its compressed state in a stuffing box. The packing material should be resilient such that it deforms under compression to conform to the interior of the stuffing box and form a tight seal against the moving shaft. The packing material should also present a low friction surface to the moving shaft and be stable under the environmental conditions to which it may be exposed.
It is also desirable that as the shaft pass through the stuffing box, it be wiped or cleaned of foreign material. Accordingly, it is preferable that along with the packing material, the stuffing box contain a material slightly abrasive to remove foreign material from the shaft without scratching or scoring the surface.
Flexible graphite has been used successfully as a packing material, as disclosed, for example, in U.S. Pat. No. 4,190,257. As used herein, the term "flexible graphite" means a material comprising essentially of graphite produced by compressing exfoliated graphite flakes to form a coherent product. Flexible graphite is favored as a packing material because its resiliency allows the packing material to conform its shape to the inner dimensions of a stuffing box and form an exact interference fit with the shaft, thus providing a seal. In addition, since it is essentially pure carbon, it has excellent thermal stability and chemical resistance. A problem with flexible graphite packing material is that it tends to extrude or flow into the clearance where the shaft passes through an opening in the stuffing box. This problem is particularly severe where the clearances are large, due, for example, to wear.
A solution to this problem is to provide a packing system which includes an annular end ring around the shaft where the shaft passes through the opening of the stuffing box. The end ring should be of a material sufficiently stiff to prevent extrusion of the end ring itself, but sufficiently resilient to form a tight seal around the shaft. The end ring functions to block or reduce the clearance between the shaft and the stuffing box and prevent packing material from extruding into the clearance.
A commonly used end ring material is braided carbon or graphite fibers. While successful to a degree, these packing materials generally contain lubricants which are not thermally stable, and volatilize upon heating, resulting in a loss of volume of the end ring. This in turn results in leaks due to loss of compression of the packing material. In addition, carbon and graphite fibers tend to fracture through movement of the shaft, producing small particles which are lost from the stuffing box, which also results in loss of volume of the end ring. These small particles also cause excessive wear to the packing material, resulting in early failure of the packing material.
It would be desirable to have a material for an end ring which is resilient such that under pressure it will conform with the interior shape of the stuffing box and the surface of the shaft, and yet have a stiffness which would inhibit flowing or extrusion. Preferably, such a material would also have abrasion resistance to prevent attrition and loss of volume of the end ring.
It is known that reinforced flexible graphite articles can be produced by pressing the flexible graphite together with a reinforcing material such as a wire mesh, or by laminating flexible graphite sheets to reinforcing polymeric or metal sheets. Examples of such articles are disclosed in U.S. Pat. Nos. 4,457,491, and 4,234,638. However, the presence of these reinforcing agents usually degrade the chemical inertness and the thermal resistance of these composites. In addition, metal reinforcements in an end ring may score the shaft and cause excessive wear.
Aylsworth, in U.S. Pat. No. 1,137,373, describes a composition in which exfoliated or expanded graphite flakes are mixed with a binder so as to coat all the exposed surfaces and the individual laminae of the graphite. The mixture is then compressed and the binder hardened or cured forming a composite comprising individual laminae of graphite bound together by a binder. The molded product may be carbonized by heating to the red heat in contact with air. These composites, although made of exfoliated graphite, do not have the microstructure of flexible graphite since each lamina is surrounded by the binder, and would have properties more similar to the binder and not have the resilience of flexible graphite materials.
In U.S. Pat. No. 4,190,257, a packing material is disclosed comprising particles of flexible graphite compressed together into a packing ring. Optionally, phenolic resins or carbon fibers may be added to act as binders or reinforcement.